feat: add extra_headers_from_state to ClientConfig

[samsja]

Summary

• Adds extra_headers_from_state field to ClientConfig that maps HTTP header names to state field names
• For each inference request, headers are dynamically built from the rollout state dict
• Enables sticky routing (e.g. X-Session-ID from example_id) without monkey-patching client classes

Usage

ClientConfig(extra_headers_from_state={"X-Session-ID": "example_id"})

Or in TOML:

[orchestrator.client]
extra_headers_from_state = {"X-Session-ID" = "example_id"}

Changes

• types.py: New extra_headers_from_state field on ClientConfig
• clients/client.py: Store config ref, add _build_state_headers(), inject headers in get_response()
• clients/openai_chat_completions_client.py: Pass extra_headers to chat.completions.create()
• clients/openai_chat_completions_token_client.py: Pass extra_headers to both fallback and token paths

Test plan

• Verify ClientConfig(extra_headers_from_state={"X-Session-ID": "example_id"}) works
• Verify headers appear on outgoing requests when state has the field
• Verify no headers added when state field is missing
• Verify no regression for clients without extra_headers_from_state

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